The theory could explain why both Skripal and his daughter Yulia were affected.

Philip Ingram, a former British military intelligence officer, told the Mail: "Using the car would explain why both Mr Skripal and his daughter got a dose."

It remains unclear when the perpetrator, or perpetrators, spread Novichok on Skripal's car. Chemical weapons experts say that Novichok can remain dangerous for a long time after being applied to a surface.

According to the Metropolitan Police, Skripal and his daughter used the car, a red BMW, to visit Salisbury city centre on the day they were poisoned.

They parked in a Sainsbury's supermarket car park, then visited a pub and a restaurant. Shortly after leaving the restaurant, they were found clearly suffering from the effects of Novichok.

Investigators this week started examining the section of the car park the Skripals used before entering The Maltings shopping centre, where they collapsed.

Neil Basu, a senior officer in London's Metropolitan Police, on Tuesday also appealed for witnesses who saw the Skripals or their car, a red BMW, in the 45 minutes before the Skripals arrived at the car park.

British Prime Minister Theresa May is preparing to set out her plans to retaliate against Russia for failing to explain itself over the poisoning. The Kremlin has repeatedly denied and openly ridiculed speculation of its involvement.